This invention relates to pneumatic runflat tires capable of being used in the uninflated or runflat condition and in particular to the ply cords used in the ply construction of the runflat tires.
Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized vehicle tires. The intent of these methods is to minimize further damage to the uninflated tire without simultaneously compromising vehicle handling for a distance between the place where the tire lost its pressure to a place, such as a service station where the tire can be changed.
Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated, or what would generally be called xe2x80x9cflat,xe2x80x9d condition. The initials EMT stand for xe2x80x9cextended mobility tire,xe2x80x9d which is an alternative designation for tires having runflat capabilities. The conventional pneumatic tire collapses upon itself when it is uninflated and is carrying the weight of a vehicle. The sidewalls of a conventional tire do not have sufficient rigidity and strength to support the weight of the vehicle without inflation. When pressure is lost, the sidewalls buckle outward in the portion of the tire where the tread contacts the ground, making the tire xe2x80x9cflat,xe2x80x9d at least in that ground-contacting region.
Various tire constructions have been suggested for pneumatic runflat tires. One approach taken has been simply to strengthen the sidewalls by increasing their cross-sectional thickness so as to resist the bending that takes place in the portions of the sidewalls that are adjacent to the ground-contacting portion of the tread. However, due to the large amounts of rubber required to stiffen the sidewall members, flexure heating becomes a major factor in tire failure during runflat operation, especially when the uninflated tire is operated at high speeds. Pirelli discloses such a runflat tire design in European Patent Pub. No. 0 475 258A1.
In general, runflat tire design is predicated upon the use of reinforcing wedge inserts inside each sidewall flex area. The wedge inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the absence of air pressure during runflat operation. U.S. Pat. No. 5,368,082, having a common assignee with the present invention, teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or inwardmost ply. While the high resistance to compression deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, the use of multiple plies and more than one reinforcing wedge insert in each sidewall has drawbacks which include the increase in tire weight and cyclical-flexure-induced heating.
In the interests of operating efficiency and performance, when operating with the tire normally inflated, the modifications to the sidewall and belt areas of the tire providing runflat operation should not adversely interact with the normal inflated operation of the tire. However, as the structure of the sidewall is stiffened with inserts and additional plies to provide runflat operation, some of the desirable operational characteristics of the tire in its inflated state are compromised.
The weight of additional sidewall plies and inserts resists the ability of the automobile suspension to allow the wheel to move in response to variations in the road surface. The increase in stiffness of the sidewall reduces the ability of the tire to comply with variations in the road surface. Both the increase in weight and radial stiffness may degrade the ride comfort and quality increasing harshness and noise perceived by the driver. Increases in tire weight associated with additional plies and inserts will also tend to reduce the performance of the automobile in straight line acceleration and deceleration. The increase in tire weight also tends to increase fuel consumption.
Clearly, the goal in runflat tire design is to provide a low-cost, light-weight tire that gives good runflat vehicle handling as well as good service life during runflat operation without compromising the durability, efficiency or performance of the tire during normal fully inflated operation.
The sidewalls of a fully inflated, conventional tire are thin, light weight and flexible compared to the sidewall of a runflat tire. The flexibility of the sidewall allows the tread area to move in a radial direction, axially inward to allow the tire to absorb bumps. As a portion of tread area moves radially inward, the adjacent sidewall radius of curvature is decreased as the sidewall bulges out. This bending of the sidewall results in a relatively small amount of bending stress because the conventional sidewall is thin. Under normal inflated operation, radial deflection is resisted by the compressive force of the air in the tire. The compressed air provides a resistance to radial deflection that is more than proportional to the deflection. For example a 10% increase in radial deflection will require more than a 10% increase in the load. In effect, the overall modulus of elasticity of a conventional tire with respect to radial deflection is not constant but increases as the deflection decreases the volume of air contained by the tire.
The reinforced sidewalls of runflat tires resist the radial deflection with a combination of compressive and bending stresses in the inflated as well as the uninflated condition. Runflat tires experience a net compressive load in the region of the sidewall closest to the road-contacting portion of the tread. Also, the bending stresses on the thickened sidewalls are such that the outer portions of the sidewalls experience tensile forces while the inner portions experience compression stresses. Thus in addition to providing the structural rigidity required for runflat operation, the reinforced sidewalls of most runflat designs also reduce the desirable radial flexibility of the tire in the inflated condition. Accordingly, the reinforced sidewalls of runflat tires tend to detract from riding comfort, due to the increased structural rigidity of the tire.
The difference in the patterns of radial stiffness between conventional and runflat tires creates several problems. A conventional tire consistently resists each increase in load with an incrementally decreasing radial deflection. With a runflat tire, whether inflated or uninflated, the response to an increase in load depends on the current level of deflection. At the initial increments of radial deflection, a runflat tire is relatively stiff as the sidewalls resist deflection with relatively little bending stress. As the load is increased, bending stresses are increased with increasing sidewall curvature and the stiffness of the tire relative to radial deflection is increased. The tendency of runflat tires to exhibit an increase in stiffness with increasing load complicates the design of the suspension and can reduce the inherent stability of the automobile. For example, in some cases, particularly in the rear wheel of an automobile while cornering, increased radial deflection of the outside rear tire can increase the load on that tire, resulting in yet more deflection in a cycle of weight transfer and deflection that results in oversteer.
The goal of providing a sidewall structure with flexibility in the inflated condition and rigidity in the uninflated condition is limited by the structural characteristics of the conventional materials used to make tires. Conventional materials have a more or less constant modulus of elasticity. A sidewall structure that uses material with a constant modulus of elasticity will tend to provide a sidewall structure that provides a less than proportional resistance to radial deflection as sidewall curvature is increased with the load or loss of air pressure. However, greater stiffness is needed during runflat operation, whereas high stiffness during normal inflated operation has adverse effects on riding comfort. What is needed is the development and use of materials that provide a variable modulus of elasticity depending on the amount of radial deflection.
The potential advantages of variable modulus methods for the refinement of the Banded Tire approach is described in U.S. Pat. No. 4,456,048, entitled xe2x80x9cDual-Modulus Banded Tire.xe2x80x9d Methods and materials for producing bands with various basic dual-modulus characteristics are disclosed. In one type, the band has a greater resistance to bending forces tending to decrease its local radius of curvature than to those tending to flatten it. In a second type, bending stiffness increases with increasing stress irrespective to the bending direction. In the third type, a two step modulus is demonstrated in which deflection is initially resisted at one rate and final deflection is resisted at a higher rate. However, the application of variable modulus approaches to the design of tire sidewalls was not considered in this prior art patent.
It is apparent that there is a need to develop a sidewall structure that provides the necessary structural rigidity in the uninflated condition while providing sufficient radial flexibility on the inflated condition to provide ride and handling characteristics equivalent to a conventional tire. In addition there is a need to minimize the additional weight of runflat tires.
It is an objective of the present invention to provide a runflat radial tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
One objective of the present invention is to provide a runflat tire with an active sidewall structure that is flexible in normal-inflated operation and stiffer in runflat operation.
Another objective of the present invention is to provide a runflat tire with multiple sidewall plies using ply cord materials providing a variable modulus of elasticity in the axially outwardmost ply layers to improve the runflat durability of the tire when the tire is uninflated.
Yet another objective of the present invention is to provide a runflat tire with multiple sidewall plies using ply cord materials providing a variable modulus of elasticity in the axially outwardmost ply layers that do not substantially increase the stiffness of the sidewall structure when the tire is inflated to provide a smooth ride in normal-inflated operation.
The present invention relates to a pneumatic runflat radial ply tire having a tread, a carcass comprising two or more radial plies, a belt structure located between the tread and the radial ply, two inextensible beads, and two sidewalls each of which is reinforced with one or more wedge inserts. The axially outwardmost ply is reinforced by cords having a modulus of elasticity that increases with strain. The increase in the modulus of elasticity of the reinforcing cords of the axially outwardmost ply can be a linear increase or a nonlinear but continuous increase. Alternatively, the increase in the modulus of elasticity of the reinforcing cords of the axially outwardmost ply might be designed to undergo one or more discontinuous rates of increase with increasing strain. Moreover, the reinforcing cords of the axially outwardmost ply might be designed to have zero to low modulus of elasticity during normal-inflated operation of the tire, while going into tension and exhibiting a high modulus of elasticity during runflat operation. The component parts of the reinforcing cords of the axially outwardmost ply might or might not operate within their respective elastic ranges during all conditions of operation of the tire.